It is known when using gas turbine engines in helicopters to attempt to split the drive train in a transmission section to reduce tooth loading and add redundancy, should one set of gear drives fail during flight. This is accomplished by having a pair of gears driven by an engine, each of the driven gears in turn driving a pair of bull pinions, which engage and drive a bull gear, which rotates the main rotor. Each set of gears (hereinafter the "gear assembly") connected between the engine and the bull gear, provides a separate drive path, each assembly capable of transmitting 100% of the engine's output to the rotor to maintain operation should one assembly fail.
The problem in designing such a redundant system is to assure even splitting of the load between the gear assemblies, as even the slightest deviation in machining tolerance in the gearing can result in a 100% split, i.e., one gear assembly transfers the entire torque load while the other assembly essentially free-wheels. This results in excessive wear and stress on the loaded assembly.
A quill shaft was used in the previous arrangement for accommodating variations in torque loading to provide even splitting between the gear assemblies. Referring to FIG. 1, a prior art gear assembly 1 is shown. The assembly uses a driven gear shaft 2, with a driven gear 3 mounted thereon, the shaft 2 supported by a pair of bearings 5 and 6. A pinion shaft 7, with a pair of bull pinions 8 and 9 mounted thereon, is also supported by a pair of bearings 10 and 11. Between these shafts 2 and 7 is disposed a quill shaft 12 having a helically splined end 13 and a straight splined end 14, which mate with splined interior sections in the driven and pinion shafts. The quill shaft allows adjustment of the gear tooth alignment between the driven and pinion shafts using a shim 15. By shimming the quill shaft, adequate torque splitting is assured, and thus, the gear assembly accommodates variations in machining tolerances.
While effective, such an assembly is complex to produce and assemble and requires multiple bearings which increase the weight penalty while reducing the assembly's useful life.